# Fantas, Eel, and Specification 5: Monoid

21 Mar 2017Good Tuesday, Fantasists! This week, we’re going to take a quick(!) look at the semigroup’s older sibling: the **monoid**. We saw last week that a `Semigroup`

type is one that has some concept of *combining* values (via `concat`

). *Well*, a `Monoid`

type is any `Semigroup`

type that happens to have a special value - we’ll call it an **identity** value - stored on the type as a function called `empty`

.

Here’s its (in my opinion, not-too-helpful) signature:

```
empty :: Monoid m => () -> m
```

*Far* more useful, I think, are the laws for how `empty`

must act for a type to be a valid `Monoid`

. We call these the identity laws:

```
// Right identity
MyType(x).concat(MyType.empty()) === MyType(x)
// Guess what this one's called?
MyType.empty().concat(MyType(x)) === MyType(x)
```

Whichever side of `concat`

we put our `empty`

, it *must* make **no difference** to the value. Let’s look at some examples of `empty`

values for our favourite semigroups. Try them on the laws above if you’re unsure of *why* they’re valid `empty`

values:

```
// ''.concat('hello')
// === 'hello'.concat('')
// === 'hello'
String.empty = () => ''
// [].concat([1, 2, 3])
// === [1, 2, 3].concat([])
// === [1, 2, 3]
Array.empty = () => []
// And so on...
Sum.empty = () => Sum(0)
Product.empty = () => Product(1)
Max.empty = () => Max(-Infinity)
Min.empty = () => Min(Infinity)
All.empty = () => All(true)
Any.empty = () => Any(false)
// BUT not every semigroup is a monoid...
First.empty = () => // ???
Last.empty = () => // ???
```

Eek, got a bit stuck at the end… `First`

and `Last`

are *not* monoids; see if you can work out why!

Ok, so,

`First`

and`Last`

actuallyaremonoids in Haskell. Thischeatis done by sneaking in an inner`Maybe`

type, where`Nothing`

becomes the`empty`

value. This actually works forany semigroupthat you want to turn into a monoid, butdon’t let Connor McBride catch you doing it…

*This is all very interesting, but what’s the point*? I’m glad you asked, imaginary reader! With a `Semigroup`

type, you can combine **one or more** values to make another, right? All a monoid does is let us upgrade that to **zero or more**. This is actually a **Pretty Big Deal™**, as we can take **any** array (including an empty array!) of monoids and `reduce`

them to one value.

*… Wait, what?*

As a surprisingly good intuition, **monoids encapsulate the logic of Array.reduce**. That’s what they do. That’s what they’re

*for*. That’s it right there. If you know how to reduce lists, then congratulations, you’re now a monoid warrior:

```
// A friendly neighbourhood monoid fold.
// fold :: Monoid m => (a -> m) -> [a] -> m
const fold = M => xs => xs.reduce(
(acc, x) => acc.concat(M(x)),
M.empty())
// We can now use our monoids for (almost) all
// our array reduction needs!
fold(Sum)([1, 2, 3, 4, 5]).val // 15
fold(Product)([1, 2, 3]).val // 6
fold(Max)([9, 7, 11]).val // 11
fold(Sum)([]).val // 0 - ooer!
```

We actually get a **double win** here. Not only do we now have a generic way to `fold`

*any* **reducible structure** (arrays, **trees**, etc) in our app with *any* `Monoid`

type (`Sum`

, `Max`

, etc), we also have an opportunity to do some *really* cool optimisations:

The thing that we didn’t explicitly mention about the semigroup laws is that *associativity* gives us an opportunity to **parallelise**. If we split a list of semigroups into chunks, `concat`

the elements of each chunk in parallel, and then `concat`

the results, we’re guaranteed to get the same result!

```
// In practice, you'd want a generator here...
// Non-tail-recursion is expensive in JS!
const chunk = xs => xs.length < 5000
? [xs] : [ xs.slice(0, 5000)
, ... chunk(xs.slice(5000)) ]
// ... You get the idea.
const parallelMap = f => xs => xs.map(x =>
RunThisThingOnANewThread(f, x))
// Chunk, fold in parallel, fold the result.
// In practice, this would probably be async.
const foldP = M => xs => fold(M)(
parallelMap(fold(M))(chunk(xs)))
// With all that in place...
// Numbers from 0 to 999,999...
const bigList = [... Array(1e6)].map((_, i) => i)
// ... Ta-da! 499999500000
// Parallel-ready map/reduce; isn't it *neat*?
foldP(Sum)(bigList).val
```

**Thanks, associativity!** By being *certain* that the `Semigroup`

and `Monoid`

laws hold for our type, we can write functions to **optimise** for different data sets, and other developers can use our API with no idea of the **wizardry** underneath!

So, monoids let us write easily-optimised and *expressive* `reduce`

operations. Pretty neat, huh? There is a tiny downside, though…

The *fiddly* part about monoids in JavaScript is that we have to pass in *type representations* (what we called `M`

). The Fantasy Land spec puts these in signatures as `TypeRep`

values, in case you’ve wondered what they were. These have to be here because JavaScript, unlike other languages, can’t *deduce* the type we’re working with, so we have to give it a friendly nudge. For example:

```
// How do we know which `empty` we want? In
// Haskell, the correct `empty` would be used
// because the type would be checked to find the
// right monoid instance in the context.
const concatAll = xs => xs.reduce(concat, empty)
// In JS, the TypeRep avoids this issue.
const concatAll_ = M => xs =>
xs.reduce(concat, M.empty())
```

This becomes more apparent when we get onto **composed monoids**. Just as we saw with semigroups, let’s imagine we want to make `Pair`

a monoid:

```
const Pair = daggy.tagged('Pair', ['a', 'b'])
Pair.empty = () => // ???
```

Remember: the `empty`

value must work for all cases, and a Pair could be made of *any* of our monoids. The solution? Pass in the `TypeRep`

s:

```
// We now have a kind of "Pair factory"!
// Pair_ :: (Monoid a, Monoid b) =>
// (TypeRep a, TypeRep b) -> (a, b) -> Pair a b
const Pair_ = (typeA, typeB) => {
const Pair = daggy.tagged('Pair', ['a', 'b'])
Pair.empty = () => Pair(typeA.empty(),
typeB.empty())
// You could write `concat` here and include
// some type-checking in its logic!
return Pair
}
// We can partially apply to get Pair
// constructors for specific types...
const MyPair = Pair_(Sum, Any)
// ... and these have valid empty() values!
// Pair(Sum(0), Any(False))
MyPair.empty()
// We can also call it directly.
// Pair(All(True), Max(-Infinity))
Pair_(All, Max).empty()
```

Some extra ugly boilerplate, but we *do* end up with the same result. We’re going to see a lot more of these `TypeRep`

values floating about, and it *is* unfortunate. Still, if you want to write type-safe JavaScript *without* all this hassle, check out PureScript!

There are loads of weird and wonderful monoids that we haven’t covered. For example, an `a -> b`

function is a monoid if `b`

is a monoid:

```
// concat :: (Semigroup b) =>
// (a -> b) ~> (a -> b) -> (a -> b)
Function.prototype.concat = function (that) {
return x => this(x).concat(that(x))
}
// Are you fed up of TypeReps yet? If you _did_
// want to implement this, you're probably better
// off setting it manually for the functions you
// are likely to concat... Sigh.
Function.prototype.empty = // result.empty()
```

Effectively, we just concatenate the results of calling *both* functions with a given argument. If this seems useless, check out Hardy Jones’ post on implementing FizzBuzz with monoids! They are *really* clever structures that, with a bit of imagination, can be spotted **everywhere** in the wild. We’ll actually come back to them time and time again in the articles to come, so get used to them!

Again, this post only touches the *surface* of what monoids can do, and I’m surprised by new examples all the time. Keep researching, keep looking for examples, see whether you could replace some of your code’s `Array.reduce`

calls with monoid folds, and start to build up a library of reusable `Monoid`

types to encapsulate your logic. **Exciting times**!

Next time, we’ll look at `Functor`

- our first step on the road to the magical `Monad`

. Until then, Fantasists,

Take care ♥